A wireless personal area network is a local area network that enables devices under a work environment of a person and devices present on the periphery thereof to be mutually interconnected.
Recently, in such a wireless personal area network, communication devices, which are compliant with the specification of IEEE 802.15.4, that are small-sized and low-priced and are capable of performing low-output digital radio communication have been used.
A network that is compliant with the specification of IEEE 802.15.4 is configured by two types of devices including an FFD (Full Function Device) and an RFD (Reduced Function Device) when the devices are broadly classified.
The FFD is a full-function built-in device having a function for approving the participation of a new device to participate in a personal area network (PAN) to which the FFD belongs and a function for defining a superframe used in communication with the other devices. Among such FFDs, a device that is present solely in each network and further has a function for determining an ID of the whole network is called a PAN coordinator.
On the other hand, the RFD is a device that does not have the participation approval function and the superframe defining function described above, which are included in the FFD, and is a function-limited-type device that has the same functions as those of the FFD except for such functions.
FIGS. 11(A) and 11(B) are schematic diagrams that illustrate topologies of networks configured by FFDs and RFDs. As illustrated in FIGS. 11(A) and 11(B), as topologies of such a network, there are topologies of a star-type network (FIG. 11(A)), a peer-to-peer network (FIG. 11(B)), and the like.
The star-type network is configured by a PAN coordinator and a plurality of FFDs or RFDs. Among all the devices, master-slave relations are formed (see Patent Literature 1). By periodically transmitting a synchronization signal (beacon) from an FFD that is a master positioned at a higher level to the FDDs or RFDs that are slaves positioned at a lower level, synchronization between the master and each slave is established, and information is transmitted and received using a TDMA (Time Division Multiple Access) system.
On the other hand, the peer-to-peer network is similar to the star-type network described above in that the network is configured by a PAN coordinator and a plurality of FFDs or RFDs but has a difference from the star-type network in that all the devices are equal, in other words, the master-slave relation is not formed between devices. Between the devices configuring the peer-to-peer network, information is transmitted and received using a CSMA (Carrier Sense Multiple Access) system.
The star-type network of the two network topologies described above will now be described in more detail. FIG. 12 is a diagram that illustrates an information transmission/reception method in a conventional star-type network.
As illustrated in FIG. 12, in the conventional star-type network, synchronization is established between a master and a slave according to a beacon signal transmitted from the master device, and an interval (Beacon Interval, BI) between beacon signals is established as a TDMA period.
The TDMA period is configured by an active period during which a slave device is operated, and information is transmitted and received and an inactive period during which the slave device is in a sleep mode.
The active period is defined as a superframe duration (SD) and is configured by a contention access period (CAP) and a contention-free access period (CFP).
The CAP is a period during which information is permitted to be transmitted and received between a master device and all the slave device that perform communication.
On the other hand, the CFP is a period during which information is permitted to be transmitted and received to/from one slave device that is assigned by the master device. During the CFP, each slave device can transmit/receive information to/from the master device only within a GTS (Guaranteed Time Slot) that is allocated thereto.
Data frames are transmitted and received between devices only during the CAP or the CFP described above.
The inactive period is a period of the BI other than the active period described above, in other words, a time other than the SD. As described above, during the inactive period, the slave device is in the sleep mode. Accordingly, as the SD occupied in the BI increases, the inactive period is shortened as that much, and the slave device is not in the sleep mode but continues to be operated.